1. Field of the Invention
This invention relates to a process for producing propylene glycol monoacetate. More particularly, this invention relates to a process for producing propylene glycol monoacetate which comprises hydroformylating vinyl acetate to .alpha.-acetoxypropionaldehyde and, then, hydrogenating the .alpha.-acetoxypropionaldehyde to propylene glycol monoacetate, and to a process for producing propylene glycol monoacetate which comprises hydrogenating .alpha.-acetoxypropionaldehyde under a defined set of conditions.
1. Description of the Prior Art
While propylene glycol monoacetate is a compound useful as a starting material for the production of propylene glycol and propylene oxide, no commercial process has yet been established for its production. Proposed thus far is a process for producing an acetic acid ester of propylene glycol by oxidation of propylene in acetic acid in the presence of a palladium catalyst (cf. British Pat. No. 1,124,862). In this process, the product compound is separated from the reaction mixture by distillation. During the distillation, however, a part of the palladium catalyst thermally degrades in the distillation vessel so that the catalytic activity cannot easily be maintained at a stable level satisfactory period of time.
There has also been proposed a process in which a compound containing 2 carbon atoms such as ethylene or acetylene is used to prepare propylene glycol monoacetate as an intermediate product which is then converted to an oxygen-containing compound of 3 carbon atoms having the formula C.sub.3 H.sub.6 O (German Offenlegungsschrift No. 2504981). More particularly, this German Offenlegungsschrift No. 2504981 teaches a process which comprises synthesizing vinyl acetate from ethylene or acetylene, hydroformylating the vinyl acetate with a gaseous mixture of hydrogen and carbon monoxide in the presence of a rhodium catalyst to give .alpha.-acetoxypropionaldehyde and hydrogenating this .alpha.-acetoxypropionaldehyde in the presence of a metal of Group 8 of Periodic Table of the Elements to provide propylene glycol monoacetate. However, the above conventional process for synthesizing propylene glycol monoacetate has the following disadvantages.
(1) In the hydroformylation of vinyl acetate, separation of the rhodium catalyst from the reaction mixture at the end of the reaction is effected by treating the reaction mixture with hydrogen at high temperature and pressure to thereby precipitate the rhodium catalyst as rhodium metal and recovering the same. However, this recovery of rhodium metal involves a complicated procedure and the accompanying loss of the rhodium catalyst represents an expense which is too great to be disregarded. Moreover, to regenerate the recovered rhodium metal, it must be treated at high temperature and pressure and the treatment requires costly equipment.
(2) When the reaction mixture resulting from the hydroformylation reaction is treated with hydrogen at high temperature and pressure, .alpha.-acetoxypropionaldehyde undergoes such undesired side reactions as decarboxylation, polycondensation, isomerization, oxidation, etc.